This project is attempting to follow the classic Free Software game business model where you share the source but keep the artwork proprietary, which is perfectly acceptable and fine. It currently sells for $3.99 and it's available for GNU/Linux, OSX, Windows, and Android.

So let this be an example to all of the new wave of indie developers. People, share you're freaking source codes under a Free License! Not only you're not losing money because of this, you'll also be getting free revisions and improvements to your own code, and you will be providing others the chance of creating something different with it.